1. Field of the Invention
This invention relates to industrial process instrumentation, and especially to fluid flow measuring apparatus of the type comprising a flow-transmitter adapted to produce an electrical signal indicating the rate of fluid flow past a vortex-generating obstruction placed in the flow path.
2. Description of the Prior Art
It has been known for many years that vortices are developed in a fluid flowing by a non-streamlined obstruction placed in the fluid flow path. It has also been known that under certain conditions the vortices are alternately shed at regular intervals from opposite edges of the obstruction to form corresponding rows of vortices. Such vortices establish a so-called Karman "vortex street", which is a stable formation consisting of two nearly-parallel rows of evenly-spaced vortices travelling with the flow stream.
In a Karman vortex street, the vortices of one row are staggered relative to those of the other row by approximately one-half the distance between consecutive vortices in the same row. The spacing between successive vortices in each row is very nearly constant over a range of flow rates, so that the frequency of vortex formation is correspondingly proportional to the velocity of the fluid. Thus, by sensing the frequency of vortex shedding, it is possible to measure the fluid flow rate.
Various proposals have been made for such flow measuring apparatus of the vortex-shedding type, and some equipment has gone into commercial use. Commonly, such apparatus comprises a rod-like vortex-shedding obstruction positioned in the flowing fluid at right angles to the direction of fluid flow. The obstruction has in many suggested arrangements been a right-circular cylinder, typically a relatively thin, elongate element as shown for example in U.S. Pat. No. 3,564,915 (FIG. 4). Other shapes have been proposed. For example, U.S. Pat. No. 3,116,639 (Bird), shows in FIG. 10 an obstruction of triangular cross-section positioned with one flat surface facing upstream. In like vein, U.S. Pat. No. 3,572,117 (Rodely) also shows the same triangular cross-section arrangement, and additionally shows a variety of different shapes.
A number of different techniques have been proposed for detecting the shedding vortices so as to develop a flow signal responsive to the shedding frequency. Thermal sensors of the so-called "hot-wire" type (i.e., thermistors, hot films, etc.) frequently have been used in vortex flow meters. The electrical resistance of such sensor elements varies with changes in the cooling rate caused by the passage of the vortices, and this resistance variation is detected by measuring the corresponding changes in current flow through the element.
Such thermal detectors have not been satisfactory for industrial applications. The sensor elements are delicate, and subject to damage or shorting-out. A potential hazard is created because the sensor element must be heated to a temperature above that of the flowing fluid, and because an electrical current must be introduced into the sensor equipment. The output signal is small and difficult to detect without highly complex electronic circuitry.
In addition, the output signal appears as a change-in-level of a non-zero current, and thus inherently presents a problem of separating the variable component from the fixed signal level. The output signal is particularly subject to noise due to cooling effects from sources other than vortices, and also is subject to extraneous variations resulting from changes in ambient conditions. Moreover, the output signal level decreases with increasing vortex frequency, and thus tends to be lost in noise signals at the higher flow rates. Protective coatings on the sensor element are quite thin in order to minimize this effect, but this, in turn, results in undesirably low resistance to wear from the flowing fluid.
As still another approach to the problem, the above-mentioned Bird U.S. Pat. No. 3,116,639 shows a relatively thin vane-like sensing element located downstream of the vortex-shedding obstruction, positioned in alignment with the direction of fluid flow and centrally located so that the spaced rows of vortices pass along opposite sides thereof. This vane-like element is said to oscillate rotatably in a twisting, torsional movement about an axis perpendicular to the fluid flow direction, in response to the pressure fluctuations of the vortices passing thereby. It is also said that the length of the vane, in the direction of fluid flow, should be equal to the vortex spacing in a row of vortices.
Various electrical transducer means are proposed in the above-mentioned Bird U.S. Pat. No. 3,116,639 for detecting the intended rotational movement of the vane, as by sensing with conventional electro-magnetic means the oscillatory twisting motion of a support shaft for the vane. This patent puts forth the notion that the vane it discloses might be made of a piezo-electric material which is strained cyclically by the passage of the vortices along its operative face to produce an alternating voltage. Piezo-electric means also are proposed as fluid-fluctuation detectors in U.S. Pat. Nos. 2,809,520 and 3,218,852. None of these prior disclosures, however, shows a practical flow meter arrangement, and developers of commercially-offered vortex-shedding apparatus have concentrated on other types of detecting devices such as previously described herein including thermal detector devices.
Electronic apparatus has of course been required with thermal detecting devices to amplify the very low pulse signals developed by the small changes in current resulting from the cooling action of the passing vortices. Typically, such electronic apparatus has been quite complex and expensive to manufacture, involving in some cases the provision of sophisticated filtering circuitry to separate the true flow signal from the background noise. The flow signals produced by such electronic equipment generally have not been well suited for industrial instrumentation purposes, and for some applications auxiliary equipment has had to be provided to establish more appropriate signal characteristics.
Accordingly, there has existed a need for improved apparatus for developing flow signals and the like adapted for use in industrial process instrumentation systems. Specifically, there has been need for improved flow-metering apparatus which is mountable directly at the flow pipe carrying the fluid to be measured, and capable of producing an electrical flow signal adapted to be sent for relatively long distances, e.g. over a two-wire transmission line to the control room of a central instrument station.